Chapter 17Chapter 17

Protein Synthesis , Protein Synthesis , Folding and ProcessingFolding and Processing

mRNA directed protein biosynthesis is amRNA directed protein biosynthesis is alsolso

called translation in molecular biology.called translation in molecular biology.

Why ?Why ?

Protein biosynthesis is a process to trProtein biosynthesis is a process to translate information from the nucleo-anslate information from the nucleo-

tide sequence of an mRNA into the tide sequence of an mRNA into the sequence of amino acids of the cor-sequence of amino acids of the cor- responding specific protein.responding specific protein.

Section OneSection One

Components Required for Protein SynthComponents Required for Protein Synthesisesis

The process of protein synthesis is one The process of protein synthesis is one of of

the most complex events in the cell.the most complex events in the cell.

It involves the coordinated participatioIt involves the coordinated participation ofn of

over 100 biomolecules.over 100 biomolecules.

Biomolecules Required in Protein SynthesisBiomolecules Required in Protein Synthesis mRNA the templatemRNA the template tRNAs transferring amino acidstRNAs transferring amino acids ribosome the location of protein synthesisribosome the location of protein synthesis aminoacyl-tRNA synthetases link tRNA andaminoacyl-tRNA synthetases link tRNA and amino acidamino acid protein factorsprotein factors ATP and GTPATP and GTP inorganic ionsinorganic ions

A mRNA works as the template in proteinA mRNA works as the template in protein synthesis.synthesis. the structure of a eukaryote mRNAthe structure of a eukaryote mRNA

Cap-----AUG-------------stop codon--------poly A Cap-----AUG-------------stop codon--------poly A 5’UTR coding region 3’UTR5’UTR coding region 3’UTR

What is the relation between the codingWhat is the relation between the coding region in mRNA and the amino acid se-region in mRNA and the amino acid se-

quence in the corresponding protein?quence in the corresponding protein?

The Genetic CodeThe Genetic Code The genetic code is the way in which the nu-The genetic code is the way in which the nu- cleotide sequence in mRNA ( or DNA ) spe-cleotide sequence in mRNA ( or DNA ) spe- cifies the amino acid sequence in protein.cifies the amino acid sequence in protein.

How ?How ?

A, G, U and C are organized into triple-nA, G, U and C are organized into triple-nuc-uc-

leotides called codons. leotides called codons.

There are 64 ( 4x4x4 ) codons.There are 64 ( 4x4x4 ) codons.

The collection of the 64 codons makes upThe collection of the 64 codons makes up the genetic code.the genetic code.

The genetic code was deciphered in 1966The genetic code was deciphered in 1966 by Nirenberg et al. by Nirenberg et al.

In the genetic code 61 codons specify 20In the genetic code 61 codons specify 20 amino acids.amino acids.

They are sense codons.They are sense codons.

There are 3 codons, UAA, UAG and UGA,There are 3 codons, UAA, UAG and UGA, which do not specify any amino acids.which do not specify any amino acids.

They are stop codons ( termination codThey are stop codons ( termination codons,ons,

nonsense codons ).nonsense codons ).

Five Features of the Genetic CodeFive Features of the Genetic Code1.1. UniversalUniversal The genetic code is used by all species,The genetic code is used by all species, from prokaryotes to human being.from prokaryotes to human being.

However some deviations are known toHowever some deviations are known to occur in mitochondria and some unicel-occur in mitochondria and some unicel- lular organisms.lular organisms.

2. Directional2. Directional The sequence of triple-nucleotide codoThe sequence of triple-nucleotide codo

ns isns is read in the direction of 5’ to 3’.read in the direction of 5’ to 3’.

The first codon of coding region of mRThe first codon of coding region of mRNA isNA is

almost always AUG, the initiation codon.almost always AUG, the initiation codon.

The last codon of the coding region of The last codon of the coding region of mRNA is one of the stop codons.mRNA is one of the stop codons.

3. commaless3. commaless The coding region in mRNA is read in a coThe coding region in mRNA is read in a co

ntinuing way without punctuation.ntinuing way without punctuation.

If there is insertion or deletion of one or tIf there is insertion or deletion of one or two wo

nucleotide(s) in the coding region of mRNnucleotide(s) in the coding region of mRNAA

frameshift mutation occurs.frameshift mutation occurs.

AUG UCG CAA GAU ACG UCCAUG UCG CAA GAU ACG UCC Met Ser Gln Asp Thr SerMet Ser Gln Asp Thr Ser

AUG CGC AAG AUA CGU CCAUG CGC AAG AUA CGU CC Met Arg Lys Ile ArgMet Arg Lys Ile Arg

AUG CUC GCA AGA UAC GUC CAUG CUC GCA AGA UAC GUC C Met Leu Ala Arg Tyr Val Met Leu Ala Arg Tyr Val

4. degeneracy4. degeneracy Because 61 codons specify 20 amino aciBecause 61 codons specify 20 amino aci

dsds multiple codons must decode the same amultiple codons must decode the same a

mi-mi- no acid.no acid.

That is called degeneracy.That is called degeneracy.

For example Leu, Ser, and Arg each is spFor example Leu, Ser, and Arg each is specified by six different codons.ecified by six different codons.

HomeworkHomework Find out other examples of codon degenerFind out other examples of codon degener

acyacy

5.5. wobblewobble

There is wobble in the process of There is wobble in the process of codoncodon

( in mRNA ) and anticodon ( in ( in mRNA ) and anticodon ( in tRNA )tRNA )

base-pairing.base-pairing.

That is the base-pairing does not strictlyThat is the base-pairing does not strictly according to the standard base-pairingaccording to the standard base-pairing rule ( A-U , G-C , T-A ).rule ( A-U , G-C , T-A ).

The wobble base pairs includes G-U, I-A,The wobble base pairs includes G-U, I-A, I-C, and I-U, and often appear in the thiI-C, and I-U, and often appear in the thi

rd rd codon-anticodon position.codon-anticodon position. 1,2, 3 1,2 31,2, 3 1,2 3 codon CAU/C AU A/C/Ucodon CAU/C AU A/C/U anticodon GU G UC Ianticodon GU G UC I

The wobble base-pairing accounts for The wobble base-pairing accounts for codoncodon

degeneracy.degeneracy.

B tRNA is the tool of transferring amiB tRNA is the tool of transferring amino no

acid.acid.There are 20 amino acids.There are 20 amino acids.There are 61 sense codons.There are 61 sense codons.There are about three dozens of prokaryThere are about three dozens of prokary

oticoticand about 50 eukaryotic tRNAsand about 50 eukaryotic tRNAs

Because of wobble base-pairing causedBecause of wobble base-pairing caused codon degeneracy 61 sense codons cancodon degeneracy 61 sense codons can be red by less than 61 tRNAsbe red by less than 61 tRNAs

It is the aminoacyl-tRNA that It is the aminoacyl-tRNA that participates inparticipates in

the protein synthesis.the protein synthesis.

The aminoacyl-tRNA synthetase The aminoacyl-tRNA synthetase catalyzescatalyzes

the linkage of an amino acid to its the linkage of an amino acid to its cognatecognate

tRNA.tRNA.

C.Ribosome is the location of protein syntC.Ribosome is the location of protein synthesishesis

A ribosome is composed of two subunits, A ribosome is composed of two subunits,

aa a large one and a small one.a large one and a small one.

Prokaryotic EukaryoticProkaryotic EukaryoticS 16SrRNA+Ps 18SrRNA+PsS 16SrRNA+Ps 18SrRNA+Ps 30S 40S30S 40SL 23SrRNA 28SrRNAL 23SrRNA 28SrRNA 5SrRNA 5.8SrRNA5SrRNA 5.8SrRNA +Ps 5SrRNA+Ps 5SrRNA +Ps+Ps 50S 60S50S 60SWhole 70S 80SWhole 70S 80S

Function of RibosomeFunction of Ribosome1.1. To hold mRNA, aminoacyl-tRNA and To hold mRNA, aminoacyl-tRNA and

translation factors in right place for ptranslation factors in right place for pro-ro-

tein synthesis.tein synthesis.2.2. To catalyze certain chemical reactionTo catalyze certain chemical reaction

ss in the process of protein synthesisin the process of protein synthesis

D . Other components for protein synthesisD . Other components for protein synthesis 20 amino acids20 amino acids dozens of tRNAsdozens of tRNAs aminoacyl-tRNA synthetasesaminoacyl-tRNA synthetases translation factors ( initiation, elongation,translation factors ( initiation, elongation, and release factors )and release factors ) ATP and GTPATP and GTP Mg2+Mg2+

Section TwoSection TwoProtein Synthesis Takes Place in Five StagProtein Synthesis Takes Place in Five Stag

eses AA activation of amino acid and synthesis ofactivation of amino acid and synthesis of aminoacyl-tRNAaminoacyl-tRNA

tRNAs are joined to amino acids to becotRNAs are joined to amino acids to becomeme

aminoacyl-tRNA in a reaction called amaminoacyl-tRNA in a reaction called amino-ino-

acylation.acylation.

Special enzymes called aminoacyl-tRNASpecial enzymes called aminoacyl-tRNA synthetases carry out the joining reactiosynthetases carry out the joining reactio

nn which is extremely specific.which is extremely specific.

nomenclature of tRNA and aminoacyl-tRNAnomenclature of tRNA and aminoacyl-tRNAa.a. cognate tRNA c. synthetase aminoacyl-tRNAa.a. cognate tRNA c. synthetase aminoacyl-tRNA ser serser serSer tRNA ser-tRNA synthetase ser-tRNASer tRNA ser-tRNA synthetase ser-tRNA

leu leuleu leuLeu tRNA leu-tRNA synthetase leu-tRNALeu tRNA leu-tRNA synthetase leu-tRNA

The aminoacylation reaction is a two-steThe aminoacylation reaction is a two-step p

reaction driven by ATP.reaction driven by ATP.

The first step is activation of amino acid.The first step is activation of amino acid.

AA + ATP + E -------------AA + ATP + E ------------- AA-AMP-E + PP AA-AMP-E + PPii

The second step is charging tRNA.The second step is charging tRNA.

tRNA + AA-AMP-E ----tRNA + AA-AMP-E ---- AA-tRNA+AMP+ AA-tRNA+AMP+EE

A cognate tRNA becomes attached to theA cognate tRNA becomes attached to the aminoacyl group through an ester bond.aminoacyl group through an ester bond.

The ester bond is formed between the acThe ester bond is formed between the acylyl

group of the amino acid residue and thegroup of the amino acid residue and the tRNA’s 3’-OH.tRNA’s 3’-OH.

Aminoacyl-tRNA synthetase has proof-Aminoacyl-tRNA synthetase has proof- reading activity.reading activity.

BB Formation of Initiation ComplexFormation of Initiation Complex ( in prokaryotes )( in prokaryotes )Initiation is the assembly of a ribosome Initiation is the assembly of a ribosome

withwith fmetfmet fMet-tRNA on an mRNA molecule.fMet-tRNA on an mRNA molecule. How?How?

1.1. IF1 and IF3 bind to a free 30S subunit.IF1 and IF3 bind to a free 30S subunit.

This helps to prevent a large subunitThis helps to prevent a large subunit binding to it without an mRNA and fobinding to it without an mRNA and fo

rm-rm- ing an inactive ribosome.ing an inactive ribosome.

2. mRNA binds to 30 S subunit by way of 2. mRNA binds to 30 S subunit by way of mRNA and 16SrRNA interaction.mRNA and 16SrRNA interaction.

There is SD sequence ( RBS , ribosomeThere is SD sequence ( RBS , ribosome binding site ) 8-13 nt upstream of the binding site ) 8-13 nt upstream of the initiation codon in prokaryotic mRNAinitiation codon in prokaryotic mRNA which base-pairs with a complementarywhich base-pairs with a complementary sequence near the 3’ end of 16SrRNA.sequence near the 3’ end of 16SrRNA.

5’ AGGAGGU 3’ SD sequence5’ AGGAGGU 3’ SD sequence 3’ UCCUCCA 5’ on 16SrRNA3’ UCCUCCA 5’ on 16SrRNA

That results in the initiation codon in thThat results in the initiation codon in thee

P site.P site.

3. With the help of IF2 and GTP the initi3. With the help of IF2 and GTP the initiator ator

fmetfmet tRNA ( fMet-tRNA )can then bind to ttRNA ( fMet-tRNA )can then bind to t

hehe initiation codon ( AUG ) on the mRNA.initiation codon ( AUG ) on the mRNA.

4. The 50S subunit can now bind, 4. The 50S subunit can now bind, which which

displaces IF1, IF2 and IF3, and GTP displaces IF1, IF2 and IF3, and GTP is hydrolyzed to GDP and Pi.is hydrolyzed to GDP and Pi.

The 70S initiation complex is The 70S initiation complex is formed.formed.

met fmetmet fmet There are two types of tRNA s , tRNA aThere are two types of tRNA s , tRNA a

ndnd metmet tRNA .tRNA .

Both of them can link a Met, and formBoth of them can link a Met, and form fmet metfmet met Met-tRNA and Met-tRNA respectively.Met-tRNA and Met-tRNA respectively. fmetfmet The Met residue in Met-tRNA is formylThe Met residue in Met-tRNA is formyl

atedated fmetfmet and fMet-tRNA is formed.and fMet-tRNA is formed.

fMet fMet fMet-tRNA is the initiator tRNA .fMet-tRNA is the initiator tRNA .

It recognizes only the initiation codon aIt recognizes only the initiation codon and nd

participates in translation initiation.participates in translation initiation.

met met

The Met residue in Met-tRNA will The Met residue in Met-tRNA will not benot be

modified.modified.

It participates in translation It participates in translation elongation.elongation.

Eukaryotic Translation InitiationEukaryotic Translation Initiation metmetThere are also two types of tRNA s in eThere are also two types of tRNA s in e

u-u- met metmet met karyotes, tRNAi and tRNAe .karyotes, tRNAi and tRNAe .

metmetBoth of them can link Met and form Met-tRNAiBoth of them can link Met and form Met-tRNAi metmet and Met-tRNAe .and Met-tRNAe . metmetMet-tRNAi is the initiator tRNA in eukaryotes.Met-tRNAi is the initiator tRNA in eukaryotes. metmetMet-tRNAe participates in translation elongatiMet-tRNAe participates in translation elongati

on.on.

Many eukaryotic initiation factors ( eIFs )Many eukaryotic initiation factors ( eIFs ) are required in eukaryotic initiation.are required in eukaryotic initiation.

Cap and poly A tail participate in the initiation.Cap and poly A tail participate in the initiation.

metmetMet-tRNAi binds to 40S subunit before mRNAMet-tRNAi binds to 40S subunit before mRNA bindingbinding

HomeworkHomework

Read eukaryote translation initiation Read eukaryote translation initiation on P.377on P.377

Draw a comparison between Draw a comparison between prokaryotic andprokaryotic and

eukaryotic translation initiation.eukaryotic translation initiation.

CC Translation Elongation Translation Elongation ( in prokaryotes )( in prokaryotes )Elongation is a process of repeated ribosoElongation is a process of repeated riboso

malmal cycles of amino acid addition.cycles of amino acid addition.

With the formation of the 70S initiation With the formation of the 70S initiation complexcomplex

elongation begins.elongation begins.

The ribosomal cycle can be divided into threeThe ribosomal cycle can be divided into three

steps, entrance ( registration ), peptide bondsteps, entrance ( registration ), peptide bond

formation and translocation.formation and translocation.

When one ribosomal cycle is completed, theWhen one ribosomal cycle is completed, the

nascent peptide is an amino acid residuenascent peptide is an amino acid residue

longer.longer.

There is another extended sense of ribosomalThere is another extended sense of ribosomal cycle, which is the cycle of translation.cycle, which is the cycle of translation.

It also has three stages, initiation, elongation It also has three stages, initiation, elongation and termination.and termination.

When such a ribosomal cycle is completed, oneWhen such a ribosomal cycle is completed, one polypeptide is synthesized.polypeptide is synthesized.

The Process of Elongation Cycle The Process of Elongation Cycle 1.1. EntranceEntrance Entrance begins when the P site is occu-Entrance begins when the P site is occu- pied by the initiator tRNA.pied by the initiator tRNA.

The complex of an elongation factor TuThe complex of an elongation factor Tu and GTP ( EFTu-GTP ) is required to de-and GTP ( EFTu-GTP ) is required to de- liver the aminoacyl-tRNA to the A site ac-liver the aminoacyl-tRNA to the A site ac- cording to the codon-anticodon base-pairing cording to the codon-anticodon base-pairing

The energy is consumed in this step by The energy is consumed in this step by hydrolysis of GTP catalyzed by EFTu.hydrolysis of GTP catalyzed by EFTu.

The EFTu-GDP is released.The EFTu-GDP is released. Another elongation factor EFTs interactsAnother elongation factor EFTs interacts with EFTu and displaces GDP.with EFTu and displaces GDP.

EFTu-GDP + EFTs-----EFTu-GDP + EFTs-----EFTuTs + GDPEFTuTs + GDP

The EFTu-GTP complex is regenerated wheThe EFTu-GTP complex is regenerated whenn

GTP displaces EFTs.GTP displaces EFTs.

EFTuTs + GTP-------------EFTuTs + GTP-------------EFTu-GTP + EFTsEFTu-GTP + EFTs

Thus EFTu has GTPase activity and EFTsThus EFTu has GTPase activity and EFTs is a GTP/GDP exchange protein.is a GTP/GDP exchange protein.

2. Peptide Bond Formation2. Peptide Bond FormationAfter aminoacyl-tRNA delivery the A and After aminoacyl-tRNA delivery the A and

PP sites are both occupied.sites are both occupied.

The two amino acids ( fMet and AA ) are The two amino acids ( fMet and AA ) are linked by forming a peptide bond.linked by forming a peptide bond.

The 23SrRNA ( in eukaryotic ribosome theThe 23SrRNA ( in eukaryotic ribosome the 28SrRNA ) catalyzes the peptide bond 28SrRNA ) catalyzes the peptide bond formation between the acyl group of theformation between the acyl group of the fMet residue and the alpha amino groupfMet residue and the alpha amino group of the next AA residue.of the next AA residue.So, protein biosynthesis is directional, froSo, protein biosynthesis is directional, fro

mm N-terminal to C-terminal.N-terminal to C-terminal.

3. Translocation3. Translocation

A complex of elongation factor G and A complex of elongation factor G and GTPGTP

( EFG-GTP ) binds to the ribosome and in( EFG-GTP ) binds to the ribosome and in

an energy-consuming step.an energy-consuming step.

The discharged tRNA from P site goes toThe discharged tRNA from P site goes to

E site and then exits.E site and then exits.

The peptidyl-tRNA is moved from the A siteThe peptidyl-tRNA is moved from the A site to P site and the mRNA moved by one codonto P site and the mRNA moved by one codon relative to the ribosome.relative to the ribosome.

EFG and GDP are released, the former beingEFG and GDP are released, the former being reused.reused.

A new codon is now present in the vacant AA new codon is now present in the vacant A site.site.

The elongation cycle is repeated until onThe elongation cycle is repeated until one e

of the stop codons ( UAA, UAG,UGA )of the stop codons ( UAA, UAG,UGA ) appears in the A site.appears in the A site.

D . TerminationD . Termination

Termination is the process of release Termination is the process of release of theof the

newly synthesized polypeptide.newly synthesized polypeptide.

Protein factors called release factors inteProtein factors called release factors inter-r-

act with stop codons and cause release act with stop codons and cause release ofof

the newly synthesized polypeptide.the newly synthesized polypeptide.

There are three RFs in prokaryotes.There are three RFs in prokaryotes.

RF1 recognizes UAA and UAG and RF2RF1 recognizes UAA and UAG and RF2 recognizes UAA and UGA.recognizes UAA and UGA.

RF3 helps either RF1 or RF2 to carry out RF3 helps either RF1 or RF2 to carry out thethe

reaction. reaction.

The release factors make 23SrRNA transThe release factors make 23SrRNA transferfer

the polypeptide to water.the polypeptide to water.

This reaction is driven by GTP and RF3 hThis reaction is driven by GTP and RF3 hasas

GTPase activity.GTPase activity.

Thus the polypeptide is released from the Thus the polypeptide is released from the ribosome.ribosome.

IF1 binds to the 70S ribosome.IF1 binds to the 70S ribosome.

The two subunits of the 70S ribosome thenThe two subunits of the 70S ribosome then dissociate. They participate in a new rounddissociate. They participate in a new round of translation initiation.of translation initiation.

There is a single release factor in eukaryoThere is a single release factor in eukaryotestes

the eRF.the eRF.

It performs the roles carried out by RF1, It performs the roles carried out by RF1, RF2RF2

and RF3 in prokaryotes.and RF3 in prokaryotes.

When a ribosome has begun translating aWhen a ribosome has begun translating ann

mRNA molecule and has moved about 70mRNA molecule and has moved about 70 - 80 nt from the initiation codon, a second - 80 nt from the initiation codon, a second ribosome can assemble on the mRNA andribosome can assemble on the mRNA and start translation.start translation.

When this second ribosome has moved alonWhen this second ribosome has moved alongg

a third can begin and so on.a third can begin and so on.

Multiple ribosomes on a single mRNA are Multiple ribosomes on a single mRNA are called polysomes.called polysomes.

In this way protein synthesis performs with In this way protein synthesis performs with high efficiency.high efficiency.

Section ThreeSection Three

Protein Folding and Posttranslational ProcesProtein Folding and Posttranslational Processingsing

The newly synthesized polypeptide mustThe newly synthesized polypeptide must undergo folding and posttranslational pundergo folding and posttranslational p

ro-ro- cessing so that it becomes the functionacessing so that it becomes the functiona

ll protein with natural conformation.protein with natural conformation.

AA Posttranslational Processing of Posttranslational Processing of Newly Synthesized PolypeptideNewly Synthesized Polypeptide

1.N-terminal processing includes removin1.N-terminal processing includes removingg

N-formyl group, or N-fMet, ( in prokaryotN-formyl group, or N-fMet, ( in prokaryotes )es )

or N-Met ( in eukaryotes ), or several amior N-Met ( in eukaryotes ), or several aminono

acid residues at the N-terminal ( in both pacid residues at the N-terminal ( in both pro-ro-

karyotes and eukaryotes ).karyotes and eukaryotes ).

In eukaryotes N-terminal signal sequenceIn eukaryotes N-terminal signal sequence ( signal peptide ) of membrane or secretory( signal peptide ) of membrane or secretory proteins are removed.proteins are removed.

There may be acetylation of N-terminal amiThere may be acetylation of N-terminal aminono

acid residue.acid residue.

C-terminal processing may also occurs.C-terminal processing may also occurs.

2. Proteolytic Processing and Protein Splicing2. Proteolytic Processing and Protein Splicing Some proteins may undergo proteolytic pro-Some proteins may undergo proteolytic pro- cessing.cessing.

The well-known example is the proteolytic pro-The well-known example is the proteolytic pro- cessing of polyprotein POMC.cessing of polyprotein POMC.

POMC is cleaved into different peptide hor-POMC is cleaved into different peptide hor- mones in different tissures.mones in different tissures.

It has been found that certain proteins in It has been found that certain proteins in prokaryotes have protein ‘ intron’.prokaryotes have protein ‘ intron’.

The protein ‘ intron’ catalyzes self-spliciThe protein ‘ intron’ catalyzes self-splicing.ng.

3. There are a lot of amino acid residue3. There are a lot of amino acid residue modification patterns :modification patterns : a. methylationa. methylation b. acetylationb. acetylation c. phosphorylationc. phosphorylation d. glycosylationd. glycosylation e. hydroxylatione. hydroxylation f. disulfide bond formationf. disulfide bond formation g. farnesylation g. farnesylation h. covalent binding of the prosthetic grouph. covalent binding of the prosthetic group and so onand so on

BB Protein FoldingProtein FoldingAs each nascent polypeptide emerges frAs each nascent polypeptide emerges fr

omom the ribosome, it begins to fold into its fithe ribosome, it begins to fold into its fi

nalnal three-dimensional structure.three-dimensional structure.

1.1. Two Models of Protein Folding:Two Models of Protein Folding:

a. hierarchical folding modela. hierarchical folding model secondary structuresecondary structuresuper-secondarysuper-secondary structurestructure tertiary structure tertiary structure

b. molten globule modelb. molten globule model hydrophobic interactionhydrophobic interactionmolten globmolten glob

uleule

2. Molecular Chaperones Participate 2. Molecular Chaperones Participate in in

Protein FoldingProtein Folding

Molecular chaperones are a kind of Molecular chaperones are a kind of proteins proteins

which help proteins to fold.which help proteins to fold.

They have following functionsThey have following functions

a. binding to the hydrophobic areas of the a. binding to the hydrophobic areas of the unfolded protein thus preventing themunfolded protein thus preventing them from abnormal aggregationfrom abnormal aggregation b . establishing an isolated environmentb . establishing an isolated environment for protein foldingfor protein folding c. promoting protein folding and de-poly-c. promoting protein folding and de-poly- merizationmerization d. in case of stress, unfolding the foldedd. in case of stress, unfolding the folded proteinprotein

Molecular chaperones are divided into twoMolecular chaperones are divided into two groups.groups. a. molecular chaperones which bind to ribo-a. molecular chaperones which bind to ribo- some such as TF, NAC.some such as TF, NAC. b. molecular chaperones which do not bindb. molecular chaperones which do not bind to ribosome such as Hsp families,PDI,andto ribosome such as Hsp families,PDI,and PPI.PPI.

Molecular chaperones’ work is driven by Molecular chaperones’ work is driven by ATP.ATP.

HomeworkHomework

Read the text on pp.383, 384,and 385.Read the text on pp.383, 384,and 385.

Summary the contents on these pages inSummary the contents on these pages in

several sentences.several sentences.

for examplefor example Hsp70Hsp701.The structure and function of Hsp701.The structure and function of Hsp702. Two helpers of Hsp702. Two helpers of Hsp703. The function of Hsp40 3. The function of Hsp40 4. The function of GrpE4. The function of GrpE

Tell the story of E coli GroEL in five seTell the story of E coli GroEL in five sentences.ntences.

CC Polymerization of Subunits,Polymerization of Subunits, Multi-subunit Protein FormationMulti-subunit Protein Formation

HbA formationHbA formationFree alpha peptide binds to nascent betaFree alpha peptide binds to nascent beta peptide and alpha-beta dimer forms.peptide and alpha-beta dimer forms.

The dimer binds to two heme molecules.The dimer binds to two heme molecules.

Two heme-containing alpha-beta dimersTwo heme-containing alpha-beta dimers interact each other and form HbA.interact each other and form HbA.

Section FourSection Four Clinical Relatives in Protein SynthesisClinical Relatives in Protein Synthesis

Protein synthesis is a regulated process.Protein synthesis is a regulated process.

The protein synthesis machinery responThe protein synthesis machinery respondsds

the environmental stimulation.the environmental stimulation.

The synthesis of ferritin, a ferrous ion (FeThe synthesis of ferritin, a ferrous ion (Fe2+)2+)

binding protein, is regulated by Fe2+.binding protein, is regulated by Fe2+.

Fe2+ binds to the 5’UTR of the ferritin mFe2+ binds to the 5’UTR of the ferritin mRNARNA

and stimulates ferritin synthesis.and stimulates ferritin synthesis.

AAMany viruses co-opt the host cell proteinMany viruses co-opt the host cell protein synthesis machinery.synthesis machinery.1.Virus mRNA is more efficiently translated1.Virus mRNA is more efficiently translated than host cell mRNA.than host cell mRNA.2. Viruses make abundant mRNA.2. Viruses make abundant mRNA.3.Some viruses can inhibit host cell mRNA3.Some viruses can inhibit host cell mRNA binding to 40S subunit.binding to 40S subunit.

BBMany antibiotics work because they selec-Many antibiotics work because they selec- tively inhibit protein synthesis in bacteria.tively inhibit protein synthesis in bacteria.

Chloramphenicol inhibits prokaryotic peptChloramphenicol inhibits prokaryotic peptidylidyl

transferase.transferase.

Streptomycin binding to 30S subunit Streptomycin binding to 30S subunit causescauses

mRNA misreading.mRNA misreading.

Tetracycline binding to 30S subunit Tetracycline binding to 30S subunit interferes interferes

with aminoacyl-tRNA binding.with aminoacyl-tRNA binding.

tyrtyrPuromycin is an analog of Tyr-tRNA and canPuromycin is an analog of Tyr-tRNA and can enter the A site during protein synthesis.enter the A site during protein synthesis. If puromycin occupies A site, translation elon-If puromycin occupies A site, translation elon- gation stops.gation stops.

Puromycin was used in the study on proteinPuromycin was used in the study on protein synthesis.synthesis.

Cycloheximide inhibits eukaryotic peptiCycloheximide inhibits eukaryotic peptidyldyl

transferase.transferase.

It is used in studies on protein metabolisIt is used in studies on protein metabolism.m.

Some bioactive substances that can inhibitSome bioactive substances that can inhibit

cell or virus protein synthesiscell or virus protein synthesis

diphtheria toxindiphtheria toxin

Diphtheria toxin is a kind of enzyme.Diphtheria toxin is a kind of enzyme.

It transfers ADP-ribose group from NAD+It transfers ADP-ribose group from NAD+

to the eukaryotic elongation factor 2 to the eukaryotic elongation factor 2 (EF2).(EF2).

That results in inactivation of EF2 and trThat results in inactivation of EF2 and trans-ans-

lation stops.lation stops. interferoninterferonWhen mammalian cell is infected by virWhen mammalian cell is infected by vir

us,us, the host cell can synthesize interferon.the host cell can synthesize interferon.

Interferon has two ways to combat virus.Interferon has two ways to combat virus.11It induces synthesis of HCI, a kind of proIt induces synthesis of HCI, a kind of pro

teintein kinase.kinase.HCI catalyzes phosphorylation of eIF2.HCI catalyzes phosphorylation of eIF2.The phosphorylated eIF2 is inactive.The phosphorylated eIF2 is inactive.Protein synthesis stopsProtein synthesis stops

22Interferon induces an enzyme for the syntInterferon induces an enzyme for the synt

hesishesis of 2’5’oligoA.of 2’5’oligoA.2’5’oligoA is synthesized in the cell.2’5’oligoA is synthesized in the cell.2’5’oligoA activates RNase L2’5’oligoA activates RNase LRNase L hydrolyzes virus mRNA.RNase L hydrolyzes virus mRNA.